Electronically tunable gain equalizer

ABSTRACT

An equalizer circuit for equalizing RF frequencies implemented as an MMIC or MIC having a series of microstrip transmission lines between an RF input terminal and an RF output terminal and having at least one shunt path comprised of a series combination of a FET able to be switched between a conducting state and a nonconducting state and a stud tuner microstrip transmission line, wherein the FET is used as a voltage variable resistor to selectively de-Q the stub tuner and thereby control the depth of the equalization curve. In a modified embodiment, artificial transmission lines which include a plurality of inductive elements and FETs are coupled to each shunt path, wherein the shunt paths may be selectively shorted by changing the state of the FETs, thereby permitting the center frequency of the equalizer circuit to be shifted as well.

FIELD OF THE INVENTION

The present invention relates to equalizer circuits and moreparticularly to an electronically tunable equalizer circuit which can befabricated either as a hybrid of microstrip transmission lines andsurface mounted resistors, or as part of a completely monolithic devicewhich includes microstrip transmission lines.

BACKGROUND OF THE INVENTION

A great deal of attention has been directed to the applications oftravelling-wave tubes (TWTs) as channel amplifiers and at this time theyare currently being used for frequency ranges from about 2 Gigahertz(GHz) to 16 Ghz. One phenomenon observed in connection with TWTs is thatthey produce higher gains and power output at the center of theiroperational band. That is, TWTs generally exhibit parabolic gain shapeswhere the gain is at a maximum at the center operating frequencies.

Many electrical systems, however, require constant gain and power outputacross their operational band. For this reason, components such as TWTs,which exhibit undesirable amplitude and/or gain variation across afrequency band, are typically "equalized" by an equalizer circuit toreduce the amplitude or gain variation to less than a few dB overrelatively broad frequency bands.

Equalizers are either passive or active circuits having a predictable,controlled attenuation slope or characteristic versus frequency.Depending upon the characteristic of the signal they equalize,equalizers fall into one of two categories: parabolic or linear. Manyconventional equalizers are of the low voltage standing wave ratio(VSWR) or absorptive type consisting of a fixed parabolic equalizer andan absorptive fine grain equalizer (FGE). Often, the bandwidth range ofconventional equalizers is too narrow to correct the gain variationsacross the frequency bands of certain devices, for example TWTs. In aneffort to equalize the gain of such devices, hybrid variable coaxialequalizers have been employed.

Hybrid variable coaxial equalizers generally consist of a maintransmission line between an RF input and an RF output, wherein thetransmission line has multiple transverse electromagnetic mode (TEM)resonant shunt branches coupled along its length. A fixed equalizer hasfixed lengths for each shunt branch or stub tuner with a lossy coatingat the end of each stub cavity. By contrast, a variable coaxialequalizer has variable stub lengths. The amount of coupling between theinput line and shunt stubs is varied, by mechanically adjusting theposition of a lossy coated plug at the end of a stub cavity, so as toalter the equalization profile and produce the desired flattening of thegain. For variable coaxial equalizers, coverage of up to two and onehalf (2.5) octaves is common with up to 20 watts of power capability. Acoaxial equalize while tunable to compensate for a variety of devicegain responses, still possesses certain disadvantages. The principaldrawbacks are large size and weight, and high cost. Typical dimensionsfor a variable coaxial equalizer operating at 6-18 Ghz, 5 watts with 15dB mid band attenuation are on the order of 1.25×1.0×0.5 inches andweight about 100 grams or more.

In co-pending application Ser. No. 07/942,728 filed Sep. 9, 1992 issuedon Feb. 1, 1994 as U.S. Pat. No. 5,283,539 and entitled MONOLITHICCOMPATIBLE, ABSORPTIVE, AMPLITUDE SHAPING NETWORK, there is disclosed aminiaturized variable equalizer circuit which can be fabricated eitheras a hybrid of microstrip transmission lines and surface mountedresistors, or as part of a completely monolithic device which includesmicrostrip transmission lines. The equalizer disclosed therein uses aband-stop filter configuration, but replaces the capacitive couplingwith resistive coupling in the form of resistive means coupled to one ormore stub tuners. The resistive means is utilized to de-Q the stubtuner, thereby permitting the attenuation profile of the equalizercircuit to be selectively determined. Specifically, the amount ofresistive coupling controls the depth of the equalization curve andchanging the length of the resonators moves the center frequency of theequalizer. While providing a smaller, lighter, and less expensivevariable equalizer, the device disclosed in the aforementionedapplication is unable to shift its loss profile to differentfrequencies.

It is an object of this invention to provide an equalizer circuit whichmay be configured as a microwave integrated circuit (MIC) or amonolithic microwave integrated circuit (MMIC) and which achievesbroadband performance without any of the aforementioned limitationsfound in conventional equalizers.

SUMMARY OF THE INVENTION

The present invention is an equalizer circuit for RF frequenciescomprising a microstrip transmission line having at least one stub tunerextending transversely therefrom and further having a voltage variableresistive means coupled to at least one stub tuner to de-Q that stubtuner, thereby selectively determining an attenuation profile for theequalizer circuit. The equalizer circuit of the present invention can befabricated either as a hybrid of microstrip transmission lines andsurface mounted resistors, or as part of a completely monolithic devicewhich includes microstrip transmission lines. In a modified embodiment,artificial transmission lines are coupled to the stub tuners to providemeans for electronically adjusting the length of the stub tuners andthereby shift the center frequency of the equalizer circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the following description of an exemplary embodiment of thereof,considered in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram depicting an equalizer circuit as a hybridcomponent in a low noise travelling wave tube driver;

FIG. 2 shows a hybrid embodiment of an equalizer circuit using a fieldeffect transistor (FET) as a voltage variable resistor;

FIG. 3 is a graph showing the frequency v. attenuation response curveswhen the FET is adjusted to give its maximum and minimum amounts ofresistance; and

FIG. 4 shows another embodiment of an equalizer circuit using anartificial transmission lines comprising inductive elements and FETs toshift the center frequency of the equalizer.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 there is shown a monolithic compatible,absorptive, amplitude shaping network (MAASNET) equalizer circuit 10.This circuit is shown in the above-noted copending application Ser. No.07/942,728. The equalizer circuit 10 is depicted as one hybrid componentin a hybrid amplifier device used as a low-noise TWT driver 11. Thehybrid driver 11 also includes a low noise amplifier (LNA) 12, anisolation amplifier (IA) 13 having a given gain, and a medium poweramplifier (MPA) 14. The output of the LNA 12 is coupled to the input ofthe MAASNET equalizer circuit 10 and the output of the equalizer circuit10 is coupled to the input of the IA 13. The output of the IA 13 iscoupled to the input of the MPA 14. The output of the MPA 14, whichcorresponds to the output of the hybrid driver 11, is coupled to theinput of a TWT 15 which receives operating potential from a power supply(LV/HVPS) 16. The LV/HVPS 16 also supplies operating potential to thehybrid driver 11. The MAASNET equalizer circuit 10 operates to equalizethe gain characteristic of the TWT 15 and to match impedences betweenthe LNA 12 and the MPA 14. In the low-noise TWT driver 11, the output ofthe LNA 12 is further amplified by the MPA 14.

Since the hybrid driver 11 of FIG. 1 includes the hybrid MAASNETequalizer 10, the TWT 15 employed can be smaller and lighter than asingle high gain TWT without gain equalization as may be implemented inthe prior art. The overall size reduction between the MAASNET equalizerand conventional equalizers, using presently available components, is onthe order of thirty percent for the type of application shown in FIG. 1.

A MAASNET equalizer circuit 10, which utilizes microstrip transmissionlines, can be incorporated into a multifunctional chip consisting ofvarious circuits. e.g., LNA and MPA. Since the MAASNET equalizer is aplanar circuit, it may be fabricated either as a monolithic microwaveintegrated circuit (MMIC) or as a microwave integrated circuit (MIC)which is a planar hybrid on a ceramic. No degradation in the equalizercircuit's performance results from the lossy nature of the circuit, evenif preceding and succeeding circuits have poor VSWR's.

Microstrip transmission lines generally comprise a conductor situatedabove or between extending conducting surfaces and are used in circuitswhere discrete devices are bonded to the circuit, and where a compactdesign is needed. Microstrip lines only approximate a TEM transmissionline. However, dispersion will not be a problem unless the circuit is tobe used for very broad bandwidth applications or it is physically manywavelengths long.

Monolithic RF circuits are usually designed on a thin semi-insulatingsubstrate of GaAs or some other semiconductor. By definition, amonolithic circuit is formed completely by a deposition method such asliquid phase epitaxy, vapor phase epitaxy, or the like. Among thecircuit elements which can be formed on monolithic substrates aretransmission lines, thin-film resistors, FETs, and transmission linestubs. Microstrip transmission lines in monolithic devices can be madewith a characteristic impedence as high as 90 ohms on a 125 μm GaAssubstrate. Since a monolithic circuit is manufactured using automatedtechniques, no final adjustment can be made once it has been produced.The circuit design must therefore rely heavily on computer modeling andoptimization. Monolithic circuits are ideal for microwave applicationsrequiring large numbers of compact units because they offer lowmanufacturing costs and good unit-to-unit repeatability.

MMIC and MIC configured equalizer circuits are approximately ten timessmaller and lighter than conventional equalizers. The interconnectionsat chip level for MMIC and MIC configured equalizers in amultifunctional circuit are such that separate RF input and RF outputconnectors (e.g., SMA type connectors for 6-18 Ghz) are obviated.Moreover, the package housings typically used in conventional equalizersare also made unnecessary. The construction details of MMIC and MICconfigured equalizer circuits using microstrip transmission lines arediscussed in significant detail in U.S. application Ser. No. 07/942,728,the disclosure of which is incorporated herein by reference.

In accordance with one embodiment of the present invention, FIG. 2 showsan MIC configured equalizer circuit 17 fabricated on a 15-mil thickalumina substrate and designed to operate in a frequency range of from 6to 18 Ghz. The design has three lossy stub tuners which are connected toa main transmission line comprised of a plurality of microstrip linescoupled in series. The microstrip lines of the :main transmission lineare approximately λ/4 apart at the center frequency, where λ isdetermined from the ratio of the speed of light to the center frequency.The dimensions and values of both the microstrip lines and the resistorswere arrived at after optimizing for a parabolic attenuation curve andIn/Out VSWR.

The input terminal 20 of the equalizer circuit shown in FIG. 2 iscoupled to the input terminal of microstrip line 22 which is 8 mils wideand 9.75 mils long. The output terminal of microstrip line 22 is coupledto T-junction 24 which has an outgoing shunt path 26. Shunt path 26 iscomprised of the series combination of stub tuner microstrip line 27,resistor 28, and open ended microstrip line 29. Stub tuner microstripline 27 is 14.5 mils wide and 15 mils long. The output terminal of line27 is coupled to the input terminal of the 55-Ω a GaAs resistor 28. Theoutput terminal of resistor 28 is coupled to input terminal ofopen-ended microstrip 29 which is 1 mil wide and 80 mils long.Microstrip line 30, which is 8 mils wide and 90 mils long, is coupledbetween T-junction 24 and T-junction 31.

The RF output terminal 40 of the equalizer circuit shown in FIG. 2 iscoupled to the output terminal of microstrip line 42 which is 5.1 milswide and 10 mils long. The input terminal of microstrip line 42 iscoupled to T-junction 44 which has an outgoing shunt path 46. Shunt path46 is comprised of the series combination of stub tuner microstrip line47, resistor 48, and open ended microstrip line 49. Stub tunermicrostrip line 47 is 14.5 mils wide and 10 mils long. The outputterminal of line 47 is coupled to the input terminal of the 200-Ω GaAsresistor 48. The output terminal of resistor 48 is coupled to inputterminal of open-ended microstrip 49 which is 6 mils wide and 20 milslong. Microstrip line 50, which is 12 mils wide and 113.3 mils long, iscoupled between T-junction 43 and T-junction 31.

In accordance with the present invention, centrally located T-junction31 has an outgoing shunt path 52 comprised of the series combination ofstub tuner microstrip line 53, field effect transistor 54, and openended microstrip line 55. Stub tuner microstrip line 53 is 8.25 milswide and 14.5 mils long. The output terminal of microstrip line 53 iscoupled to the source electrode of field effect transistor (FET) 34. Thedrain electrode of FET 54 is coupled to the input terminal of open-endedmicrostrip line 55, which, for example, is 2 mils wide and 70 mils long.FET 54 acts as a passive device in that it can not provide any gainunless a voltage bias is applied between the drain and sourceelectrodes. When a voltage below pinchoff is supplied to the gateelectrode of FET 54 and its drain and source are held at ground, FET 54acts as a voltage variable resistor with the resistance value inverselyproportional to the gate control voltage. FIG. 3 shows the frequency v.attenuation response of the equalizer circuit embodiment shown in FIG. 2with the voltage supplied to FET 54 varied to provide maximum andminimum amounts of resistance.

Although it is contemplated that resistors 28 and 48 may be replacedwith FETs to provide variable resistance at those locations as well,coupling a single FET to a centrally disposed transverse shunt path ispreferred for several reasons. First, the control voltage is relativelyeasy to generate. If several FETs are used, each requires a differentcontrol voltage, thereby necessitating a complex drive circuit. Second,varying the values of resistance proximate the input and output of theequalizer circuit affects the return loss of the equalizer. When onlythe center resistance is varied, however, the return loss remainssubstantially constant across the range of attenuation shapes. Finally,FETs used as variable resistors are not pure resistances, but have bothinductive and capacitive parasitics which can adversely affect theperformance of the equalizer. It will be readily appreciated that suchparasitics are minimized when only a single FET is used. Beneficially,parasitics present in the center of the equalizer structure can be atleast partially absorbed.

FIG. 4 shows an alternate embodiment of an equalizer circuit, whereinlike numerals are used to denote like parts. In the embodimentillustrated in FIG. 4, the shunt paths 26, 46, and 52 are modified inthat the open circuit configuration provided by open-ended microstriplines 29, 49, and 55 in FIG. 2 is replaced with a short circuitconfiguration in order to permit the center frequency of the equalizerto be shifted. For this purpose, artificial transmission lines 60, 70and 80 are connected to microstrip lines 29, 55, and 49 respectively.

Artificial transmission line 60 comprises inductive elements 61, 63, 65and 67, which are coupled together in series to microstrip line 29.Adjacent inductive elements are coupled together at a T-junction. To theremaining leg of each respective T-junction is coupled a correspondingFET 62, 64 or 66. The sources of FETs 62. 64, and 67 are grounded.

Artificial transmission line 70 comprises inductive elements 71, 73, 75and 77, which are coupled together in series to microstrip line 55.Adjacent inductive elements are coupled together at a T-junction. To theremaining leg of each respective T-junction is coupled a correspondingFET 72, 74 or 76. The sources of FETs 72, 74, and 76 are grounded.

Artificial transmission line 80 comprises inductive elements 81, 83, 85and 87, which are coupled together in series to microstrip line 49.Adjacent inductive elements are coupled together at a T-junction. To theremaining leg of each respective T-junction is coupled a correspondingFET 82, 84 or 86. The sources of FETs 82, 84, and 86 are grounded.

Each FET of the respective artificial transmission lines is used as apassive device by switching it between its on and off state. In the offstate, the FET behaves much like a very large resistance in parallelwith a shunt capacitance. In the on state, however, the resistance ofthe FETs is very low. Thus, in the on state, the FETs may be used toshorten the artificial transmission line and hence shorten the stubtuner, thereby increasing the equalizer's resonant frequency.Accordingly, it will be appreciated that in the illustrative embodiment,the center frequency of the equalizer may be shifted by switching FETs66, 76, and 86 to the on state. In like fashion, the center frequencycan be further shifted by switching FETs 64, 74, and 84 to the on state.The greatest shift is obtained by switching all of the FETs to the onstate. It will therefore be readily apparent that by choosing how muchof the artificial transmission line to short, the center frequency canbe shifted to accommodate a variety of attenuation profiles, therebyallowing it to be used to compensate for the responses of differentdevices.

Many advantages of the present invention are attributable to itsmonolithic configuration. The parabolic response can be linearized overa frequency band by increasing the number of the circuit elements. Overits linear range, the equalizer can be used in the input matchingcircuit of solid state amplifiers to obtain flat gain response. This isaccomplished by gain equalization of 6 dB/octave gain roll-off slope ofthe active devices. It is not necessary to increase the number ofcircuit elements to increase the equalizer circuit's attenuation levelor profile. The attenuation range or profile of the equalizer circuitcan be changed simply by changing the resistance of FET 54 and/orselecting different resistance values for resistors 28 and 48, and byreoptimizing the microstrip line widths and lengths to fit the requiredresponse curve. Further, in accordance with an embodiment of theinvention exemplified in FIG. 4, the center frequency can be shifted toaccommodate more than one type of device response by selectivelyshorting out part or all of the artificial transmission lines.

The equalizer circuits of the present invention do not impact the powerconsumption of TWTs or other devices to which they may be coupledbecause FETs used as passive devices consume little power. Additionally,either circuit can be implemented as an MMIC, thereby providing verysmall equalizers in large quantities at low cost. Additionally, eitherequalizer circuit may be configured as a portion of a multifunction MMICwhich could provide gain shaping, phase calibration, amplitudecalibration, and gain.

It will be understood that the embodiments described herein, includingthe resistor values given and lengths of the various microstrip lines,is merely illustrative and that a person skilled in the art may makemany variations and modifications to the described embodiments utilizingfunctionally equivalent elements to those described. Any variations ormodifications to the invention just described are intended to beincluded within the scope of said invention as defined by the appendedclaims.

What is claimed is:
 1. An electrically tunable equalizer circuit for RFfrequencies of various attenuation profiles, wherein said tunableequalizer has a predetermined center frequency, said tunable equalizercircuit comprising:a main microstrip transmission line having an RFinput port and an RF output port; a stub tuner disposed proximatelycentrally on said main microstrip transmission line; shifting means forselectively shifting said predetermined center frequency to compensatefor said various attenuation profiles; and variable resistive meanscontained within said shifting means, wherein said variable resistivemeans is coupled to said stub tuner for providing a voltage variableresistance, said various resistive means being adapted to de-Q said stubtuner and thereby selectively determine an attenuation profile for saidequalizer circuit.
 2. The equalizer circuit according to claim 1 whereinsaid equalizer circuit is fabricated as a hybrid circuit on a substrate.3. The equalizer circuit according to claim 2, wherein said variableresistive means includes a metal-semiconductor field-effect transistor.4. The equalizer circuit according to claim 1, wherein said equalizercircuit is fabricated as part of a monolithic device.
 5. The equalizercircuit according to claim 4, wherein said variable resistive meansincludes a metal-semiconductor field-effect transistor.
 6. The equalizercircuit according to claim 1, further comprising a short circuitedmicrostrip transmission line coupled to said variable resistive means.7. The equalizer circuit according to claim 1, wherein said shiftingmeans includes an artificial transmission line coupled to said variableresistive means, said artificial transmission line including at leastone inductive element and a field effect transistor.
 8. The equalizercircuit according to claim 1, wherein said equalizer further comprises:asecond stub tuner extending transversely from said main microstriptransmission line proximate said RF input port; a third stub tunerextending transversely from said main microstrip transmission lineproximate said RF output port; means coupled to said second stub tunerto de-Q said second stub tuner, thereby selectively determining anattenuation profile for said equalizer circuit; and means coupled tosaid third stub tuner to de-Q said third stub tuner, thereby selectivelydetermining an attenuation profile for said equalizer circuit.
 9. Theequalizer circuit according to claim 8, wherein said means to de-Q saidsecond stub tuner and said means to de-Q said third stub tuner areresistors.
 10. The equalizer circuit according to claim 8, wherein saidequalizer circuit is fabricated as part of a monolithic device and saidmeans to de-Q said second stub tuner and said means to de-Q said thirdstub are thin-film resistors.
 11. The equalizer circuit according toclaim 8, wherein said shifting means includes:a first artificialtransmission line coupled to said variable resistive means; a secondartificial transmission line coupled to said means to de-Q said secondstub tuner; and a third artificial transmission line coupled to saidmeans to de-Q said third stub tuner, wherein said first, second, andthird artificial transmission lines include at least one inductiveelement and a field effect transistor.
 12. An equalizer circuit for RFfrequencies, wherein said equalizer circuit has a predetermined centerfrequency, said equalizer circuit, comprising:a main microstriptransmission line having an RF input port and an RF output port; a firsttransverse microstrip transmission line forming a first stub tunerextending transversely from said main microstrip transmission lineproximate a central line bisecting said main microstrip transmissionline; means coupled to said first stub tuner to de-Q said first stubtuner, said means to de-Q said first stub tuner being adapted to providea variable resistance; a second transverse microstrip transmission lineforming a second stub tuner extending transversely from said mainmicrostrip transmission line proximate said RF input port; means coupledto said second stub tuner to de-Q said second stub tuner; a thirdtransverse microstrip transmission line forming a third stub tunerextending transversely from said main microstrip transmission lineproximate said RF output port; and means coupled to said third stubtuner to de-Q said third stub tuner; and shifting means coupled to saidmeans to de-Q said first stub tuner, said means to de-Q said second stubtuner and said means to de-Q said third stub tuner for selectivelyshifting said predetermined center frequency.
 13. The equalizer circuitaccording to claim 12, wherein said means to de-Q said first stub tuneris adapted to provide a voltage variable resistance.
 14. The equalizercircuit according to claim 12, wherein said means to de-Q said firststub tuner includes a metal semiconductor field effect transistor. 15.The equalizer circuit according to claim 12, wherein said shifting meansincludes:a first artificial transmission line coupled to said means tode-Q said first stub tuner; a second artificial transmission linecoupled to said means to de-Q said second stub tuner; and a thirdartificial transmission line coupled to said means to de-Q said thirdstub tuner, wherein said first, second, and third artificialtransmission lines include at least one inductive element and a fieldeffect transistor.
 16. The equalizer circuit according to claim 15,wherein each of said artificial transmission lines comprises a pluralityof inductive elements coupled in series and a plurality of field effecttransistors, a respective field effect transistor being coupled to acorresponding junction of adjacent inductive elements, whereby saidartificial transmission line may be selectively shorted by applyingvoltage to said field effect transistors to shift said center frequencyof said equalizer circuit.